Fix off by one date error

src/lib/components/calendar.svelte

@@ -27,9 +27,13 @@
     let currentYear = $state(parseInt(year));
     let currentMonth = $state(parseInt(month));
 
+    function toISODate(year: number, month: number, date: number) {
+        return `${year}-${String(month + 1).padStart(2, '0')}-${String(date).padStart(2, '0')}`;
+    }
+
     function getEntryForDay(year: number, month: number, date: number) {
         if (!entries) return undefined;
-        const dateStr = new Date(year, month, date).toISOString().split("T")[0];
+        const dateStr = toISODate(year, month, date);
         return entries.find((e) => e.date.split("T")[0] === dateStr);
     }
 
@@ -44,7 +48,7 @@
             date: date.getDate(),
             month: date.getMonth(),
             year: date.getFullYear(),
-            iso: date.toISOString().split('T')[0],
+            iso: toISODate(date.getFullYear(), date.getMonth(), date.getDate()),
             dayName: headers[date.getDay() === 0 ? 6 : date.getDay() - 1],
             isCurrentMonth,
             isToday: date.toDateString() === today.toDateString(),